The present invention relates to a color copier, color printer or similar color image forming apparatus.
It is a common practice with a color image forming apparatus to prevent the mixture of colors by causing each developing unit to move into and out of contact with an image carrier independently of the others, causing only a developer carrier to move in a reciprocating motion, or selectively rotating a developing sleeve forward or backward. In an apparatus of the type moving each developing unit into and out of contact with an image carrier as mentioned above, it is likely that the image carrier receives an instantaneous impact load in the event when a plurality of toner images of different colors are combined on the image carrier or when a toner image is transferred to a transfer body. Then, the load acting on the image carrier changes to bring about the misregistration of toner images of different colors and the dislocation of the toner image, noticeably degrading the image quality. To eliminate this problem, use may be made of an eccentric cam for causing the load to act on the image carrier slowly when each developing unit is brought into and out of contact with the image carrier independently of the others, as proposed in the past.
Another conventional color image forming apparatus has an image carrier for forming an electrostatic latent image thereon, a plurality of developing units facing the image carrier and each storing a developer of particular color, and driving means for selectively rotating a developing sleeve included in each developing unit forward or backward to form or cancel a magnet brush of a developer. This type of apparatus is not provided with any measure against the changes in the load acting on the image carrier and, therefore, suffers from the misregistration of color components and the dislocation of a composite image.